The present invention relates to an image recorder for recording images on sheets and, more particularly, to a mechanism installed in a sheet feeding device of such an image recorder for feeding sheets one by one out of a sheet cassette.
In an electrophotographic copier, facsimile machine, laser printer or similar image recorder, a latent image representative of a document image is electrostatically formed on a photoconductive element or similar image carrier and then developed by a developer such as a toner. The developed image or toner image is transferred to a sheet which is fed from a sheet cassette of a sheet feeding device which is loaded with a stack of sheets. The sheet with the toner image is transported to a fixing station to fix the toner image. The sheet feeding device includes a mechanism for driving sheets one by one out of the sheet cassette as stated above. The sheet feed mechanism usually includes a cassette elevating device which is so arranged as to raise the sheet cassette until a pick-up roller exerts a predetermined pressure on the sheets. While the pick-up roller drives the sheets outward, a feed roller and a reverse roller cooperate to separate the uppermost sheet from the others and feed it toward the photoconductive element on which the toner image is formed. A support arm is affixed to the sheet cassette at one end thereof. An arm is rotatably mounted on the other end of the support arm. The reverse roller is mounted on a shaft which is in turn mounted on one end of the arm. A reverse motor is drivably connected to the reverse roller via a drive gear, driven gear, etc. A spring is anchored to the other end of the arm to constantly bias the arm toward the sheet cassette. The pressure exerted by the pick-up roller on the sheets, i.e., pick-up roller pressure, is controlled to a predetermined value by operating the cassette elevating mechanism. The pressure exerted by the reverse roller on the feed roller, i.e., reverse pressure, is generated by the particular angle of the arm to the cassette, the gear ratio of the gear elements, and the force of the spring.
A problem with the prior art sheet feed mechanism stated above is that the structure is complicated and the production cost is high, because the pick-up roller for driving the sheets and the cassette elevating mechanism for setting up a predetermined roller pressure are essential. Another problem is that the gears, arm and spring are located between opposite side walls of the image recorder and obstruct maintenance, manipulations for setting the reverse pressure, etc. Furthermore, since the reverse roller is constantly subjected to a force which tends to drive it in the reverse direction, an anti-reverse clutch has to be provided for preventing the feed roller from being reversed while it is not driven, aggravating the complicated structure of the mechanism.